Method of electrolytically sulfiding ferrous parts in a thiocyanate bath



United States Patent 3,475,291 METHOD OF ELECTROLYTICALLY SULFIDING FERROUS PARTS IN A THIOCYANATE BATH Jacques Jean Caubet, 79 Rue Neyron,

Saint-Etienne, Loire, France 'No Drawing. Filed Sept. 27, 1966, Ser. No. 582,269 -Claims priority, application France, Oct. 15, 1965,

Int. (:1. 3323b 5/50 us. or. 205 -39 4 Claims ABSTRACT or THE DISCLOSURE In the French Patent No. 1,406,530, there has been described a method of treatment of metallic surfaces with the object of improving their qualities of friction and resistance to wear, consisting of introducing a nonmetallic element into the surface of the treated metallic parts by electrolysis in a bath of molten salts, the part treated constituting the anode. For the constitution of the bath of molten salts, cyanates have been more particularly proposed, especially potassium cyanate melted at 350 C.

With baths of this kind there is obtained an incorporation of the elements carbon (C) and nitrogen (N) into the surface of the part treated, which increases the surface hardness and reduces the coefficient of friction.

The present application has for its object an improvement in this method.

This improvement is characterized in that the bath of molten salts in which the metal part is immersed and subjected to electrolysis, is a mixture of thio-cyanates, and more particularly a mixture of sodium thiocyanate and potassium thiocyanate.

It has been observed that with this mixture of thiocyanates, a number of advantages were obtained. In the first place, there is obtained the incorporation in the surface of the metal of the element sulphur (S), especially in the form of sulphide, which is favourable to the improvement in the qualities of friction.

Also, with the mixture of salts, the temperature of the molten salt bath can be considerably reduced, to 150- 200 C., which enables parts to be treated, especially of special steels, without reducing their hardness.

The most advantageous results are obtained by using as the molten salt bath a mixture of thiocyanates in a eutectic proportion, namely 25 of sodium thiocyanate and 75 of potassium thiocyanate. This mixture has the property of melting at 125 C., which is a relatively-low temperature as compared with that of the usual molten salt baths, and enables treatment to be carried out at 150-200 C., as indicated above.

To the bath of molten thiocyanates there may be added a ferricyanide, a ferrocyanide or a mixture of these two. This addition is made in small proportions; an advantageous proportion is 0.9% of ferricyanide and 0.1% of ferrocyanide. This addition has the advantage of facilitating the application of the method in the case of treatment of parts of complicated shapes.

3,475,291 Patented Oct. 28, 1969 In addition, the treatment is still further improved by carrying out at the base of the crucible which serves as a cathode, an injection by blowing van inert gas such as nitrogen or a reducing gas such as hydrogen into the bath of molten salts. With this blowing injection, the anode-cathode voltage is reduced for the same current density.

The method according to the invention is applicable to all metal parts, the surface of which contains a large proportion of iron, nickel or both these metals. It gives particularly important results with steel'parts when they have been previously case-hardened and tempered or nitrided.

Considerable improvements in surface quality have also been obtained with parts of various metals by proceeding, beforetreatment, to coat these parts with a thin layer of iron or nickel, by. galvanization.

In addition, by means of a previous deposit of this kind, of a few microns in thickness, of a homogeneous coating of iron or nickel, the action of electrolysis is I made uniform and the surface condition of the part treated is improved, even on steel parts.

EXAMPLES Example 1 tive pole. The electrolysis is carried out for 15 minutes with a current density of 2.5 amperes per square decimeter with an anode-cathode voltage of 0.8 to 2 volts.

The part thus treated is washed for 2 hours in running water to dissolve the dried salts of the bath, and then brushed with a metal brush and subjected to the Faville test. Placed between two jaws cut to a V at 90, of 16N6 steel case-hardened, tempered, untreated, this part is rotated at 350 rpm. whilst there is applied to the jaws a gripping force which increases at the rate of 5 kilograms-force per second.

The test is stopped either when the part seizes or when the creep of the test-piece which heats-up compensates for the approach of the jaws. The index of quality is supplied by the value f fdt in which t represents the duration of the test.

Whereas an untreated test sample generally seizes-up in the first few seconds of the test, the test sample treated with the above-mentioned bath creeps without seizure after seconds. The index of quality exceeds 30,000 kg.-f./sec.

Upon examination by electronic diffraction, there is observed the presence in the surface of the part of iron sulphide FeS over a depth of about 15 microns. The qualities of friction obtained appear to be attributable to the presence of this iron sulphide.

Example 2 The surface condition is improved by adding to the eutectic bath of Example 1:

0.1% of potassium ferrocyanide; 0.9% of potassium ferricyanide.

In the bath thus constituted, there are treated by electrolysis with a current density of 1.5 amperes per square decimeter for 15 minutes, gears having a pitch diameter of 180 mm., module 12, the treated gears serving as the cathode. These gears have remarkable friction qualities.

Example 3 In a bath of molten salts identical with that of Example 2, there is treated a test sample of steel previously treated by nitridation to a depth of 0.5 mm.

The electrolytic treatment carried out under the same conditions as Examples 1 and 2, has the result of increasing three-fold the quality index of the Faville test.

Example 4 In order to improve the surface condition while reducing the potential difference between anode and cathode, there is blown-in at the base of the crucible a mixture of nitrogen and hydrogen comprising 60% of nitrogen and 40% of hydrogen by volume.

During the treatment of a Faville test sample in a eutectic bath of 2 kg. contained in a crucible of 100 mm. in diameter, with a current density of 2.5 amperes per square decimeter, the potential difference between anode and cathode passes from 1.2 to 1 volt when 1 litre of the gaseous mixture is blown-in per minute.

Example A part made of case-hardened tempered steel treated as in Example 1, has irregularities of surface probably due to local differences of potential. The roughness of the part is increased in consequence of the treatment by electrolysis; it passes from 3 to 5 microinches (centerline average) or the AFNOR Standard (French Standards Association) PNE 05-012, to 25-30 microinches CLA.

By proceeding before the electrolysis to give a coating on the part to be treated by an electrolytic deposit of 3 to 4 microns in thickness, of iron or an alloy comprising more than 50% of iron, the final roughness after electrolytic treatment in the bath of molten thiocyanates is only to 12 microinches CLA.

What I claim is:

1. A method of treating the surface of a part of ferrous metal in order to reduce its friction and improve its wear resistance, comprising the steps of immersing the part in a bath of molten salts consisting essentially of a eutectic mixture of 25% sodium thiocyanate and potassium thiocyanate, passing an electric current through the part and the bath with the part connected as the anode, and maintaining the temperature of the bath at 150 to 200 C.

2. A method as claimed in claim 1, in which the bath contains about 0.9% potassium ferricyanide and about 0.1% potassium ferrocyanide, by weight of the whole.

3. A method as claimed in claim 1, and depositing on the part a coating of a member selected from the group consisting of iron and iron base alloy, prior to immersion in the bath.

4. A method as claimed in claim 1, and blowing into the bath from the bottom of the bath a gas selected from the group consisting of nitrogen and hydrogen and a mixture thereof.

References Cited UNITED STATES PATENTS 1,953,647 4/1934 'Darrah 204- 1,961,520 6/1934 Malcolm 148-l5.5 2,231,009 2/1941 Holt 14815 2,339,223 1/1944 Holt 148-155 2,707,159 4/1955 Foucry et a1. 14828 3,331,708 7/1967 Buitkus 14815.5

FOREIGN PATENTS 1,023,696 12/1952 France. 1,406,530 5/1964 France.

697,224 11/1964 Canada.

ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 

